1. Field of the Invention
The present invention relates to a motor control apparatus used for controlling a DC motor and an image forming apparatus including the motor control apparatus.
2. Description of the Related Art
In forming a toner image on a sheet of paper, some copying machines or printers form a toner image on a photosensitive drum, transfer the toner image onto an intermediate transfer belt, and transfer the toner image from the intermediate transfer belt onto a sheet. In order to obtain a high quality image, some of such copying machines or printers employ different motors for driving the photosensitive drum and the intermediate transfer belt. According to such a configuration, when a peripheral speed of the photosensitive drum is different from that of the intermediate transfer belt, since the photosensitive drum contacts the intermediate transfer belt at a transfer position, a surface flaw may be made on the photosensitive drum or the intermediate transfer belt.
Such a problem does not occur when the photosensitive drum and the intermediate transfer belt are driven at a controlled target speed. However, when the photosensitive drum and the intermediate transfer belt are driven by different motors (e.g., DC motors), since it is difficult to control the start-up of the motors from a standstill state until the motors obtain a target angular velocity, the motors will have different start-up characteristics (e.g., overshoot). Unless the motors have similar start-up characteristics, the photosensitive drum and the intermediate transfer belt will have different peripheral speeds at the time the motors are started.
In order to speedily start-up the DC motor with a small overshoot, Japanese Patent Application Laid-Open No. 7-039181 discusses a method for increasing a pulse width modulation (PWM) signal, which is used for increasing the speed of the motor from a low duty cycle by a predetermined rate. Further, Japanese Patent Application Laid-Open No. 2007-156238 discusses a method for controlling the clock frequency of the DC motor. When the motor is started, the clock frequency of the DC motor is controlled at a frequency lower than a target frequency. After the motor is started, the clock frequency is increased to the target frequency. However, even if the DC motor is controlled so that the control value is increased to a target control value when the DC motor is started and accelerated, although the angular velocity of the DC motor is increased linearly for some time, it shows a non-linear transition when the control value reaches near the target angular velocity.